Optical disk systems for storing and reading information are known. The information is stored on a disk by causing a mark or deformation on the disk with a laser. To read the information from the disk, a laser beam at low power is positioned and focused on a disk and reflectance from the disk is read. Typically, for positioning the laser at various locations on the disk, the disk is rotated and a radial actuator assembly is translated along a radius of the disk.
Similar positioning assemblies are known for magnetic disk systems. Such systems differ essentially in that information is stored in a magnetic form rather than an optical form. Typically, the magnetic disk is rotated and an assembly is translated in a radial direction. In at least some respects, positioning methods are different for optical disks. Typically, bit sizes are smaller for optical disks, and therefore actuator response frequencies must be higher. Both magnetic and optical disks typically contain information in discrete, spiral or concentric circular tracks on the disks. Most optical disks have spiral tracks. To read information at a particular location, an assembly must be positioned directly in line with the correct portion of the track. The disk is also rotated so that the information within the given track is positioned over the actuator assembly.
The width of a track in a typical magnetic floppy disk is on the order of 200 microns. The track within a magnetic hard disk is typically on the order of 10 microns. Optical disks have a track width on the order of 1.5 microns. Accordingly, the radial positioning accuracy for optical disk carriages required to obtain correct reading of information is greater than that required for magnetic disk systems.
Numerous carriage actuator apparatuses for magnetic disk systems are known. For example, U.S. Pat. No. 4,012,778 to Johnson (Mar. 15, 1977), discusses a linear actuator which translates magnetic assemblies by means of a drive shaft and two rollers. Each assembly is supported by two parallel guide rails.
U.S. Pat. No. 4,646,182 to Sakurai (Feb. 24, 1987), discusses a carriage assembly for a magnetic disk apparatus. This apparatus includes a carriage which rides on two parallel guide rods on some bearing means. Also included is an elastic member between the carriage bearing means so that if the rods are not parallel, any deviation can be absorbed by elastic deformation.
U.S. Pat. No. 4,427,905 to Sutton (Jan. 24, 1984), discusses a carriage assembly for a magnetic data storage system. The carriage travels on two parallel support rods and the assembly is designed so that the center of gravity of the carriage assembly is approximately aligned with the center of force used to translate the carriage assembly on the support rods.
As indicated above, the performance requirements for magnetic systems are less than those for optical systems because the track width and bit size in optical systems is substantially less. Therefore, amounts of actuator assembly inaccuracy and jitter acceptable for magnetic systems are too great for optical systems. Because of the smaller bit size, a much higher tracking accuracy is needed in an optical system. This higher tracking accuracy can be achieved by providing a higher bandwidth (the frequency at which the transfer function is equal to one) in the tracking loop, i.e. the feedback system for positioning the actuator with respect to the information track in the disk. Known one-stage tracking systems have achieved bandwidths only up to about 500 hertz. The present apparatus can achieve a bandwidth of up to about 2 khz and up to 3 khz if no major phasenegative resonances occur below 10 khz.
Moreover, optical systems have several design requirements not associated with magnetic systems. For example, optical systems include a lens on the actuator for focusing the laser beam on the optical disk. Additionally, optical systems require an apparatus for moving the lens for its focusing function. Actuators in optical systems must also be designed to allow a path for light to travel from a light source to the lens.
Some optical information systems include two-stage radial movement mechanisms for transporting a lens with respect to a disk. The first stage controls the large, slow movements of the actuator (so-called "coarse tracking") and the second stage controls the fast, fine movement of the objective on the actuator (so-called "fine tracking"). While these systems can be highly accurate, they are relatively expensive to build and complicated to construct because they have two separate radial transport mechanisms.
Other optical systems have one-stage radial movement. For example, U.S. Pat. No. 4,443,721 to Jansen (Apr. 17, 1984), describes a device for translating an objective in which the assembly is supported on two parallel guide rods which are magnetizable and which form part of the stator yoke.
U.S. Pat. No. 4,545,046 to Jansen, et al. (Oct. 1, 1985), discusses an optical recording device including an opto-electronic system for converting the reflected optical beam into an electrical modulation in which the opto-electronic system is stationarily positioned with respect to the parallel guide rods on which the slide is translated.
Jansen, U.S. Pat. No. 4,607,913 (Aug. 26, 1986) describes an electrodynamic device for translating a lens in an optical disk system. This device includes a pair of radial drive coils for the slide and a pair of radial drive coils for the objective which are dimensioned to apply radial forces to the slide and objective which are proportional to their respective masses.
Some optical information systems, known as magneto-optic systems, store magnetic information on a disk which can be written and read optically. Such systems typically have a strong magnet for writing magnetic information to the disk which is positioned above the focusing lens which focuses an optical beam on the information disk. A problem with such systems is that this magnet can interfere with electromagnetic mechanisms for focusing the lens and translating the actuator.
While devices are known for achieving radial movement of a lens in optical disk systems, there is a need for actuator carriages having improved stability to allow for higher-frequency movement of focusing lenses. Improved stability allows for higher acceleration to reduce access time and for more accurate positioning so that one-stage systems can be used. There is also a need for lens-positioning devices for optical information systems which can be used in magneto-optic systems without substantial interference to electromagnetic lens focusing systems.